1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same. More particularly, it relates to a semiconductor device having a superjunction structure with p-type pillar layers and n-type pillar layers buried in a drift layer laterally and alternately, and a method of fabricating the same.
2. Description of the Related Art
A vertical power MOSFET has an on-resistance greatly dependent on the electrical resistance in a conduction layer (drift layer) thereof. The electrical resistance in the drift layer is determined from its impurity concentration, and the on-resistance can be lowered as the impurity concentration is made higher. A higher impurity concentration, however, lowers the breakdown voltage across a PN junction formed by the drift layer together with a base layer. Accordingly, the impurity concentration can not be made higher than the limit determined in accordance with the breakdown voltage. Thus, there is a tradeoff between the device breakdown voltage and the on-resistance. An improvement in tradeoff is one important subject matter to provide a semiconductor device of low power dissipation. The tradeoff has a limit determined from device material, and exceeding the limit is a way to realize a low-on-resistance semiconductor device.
As an example of the MOSFET to solve the problem, there is a known structure, which is referred to as a superjunction structure with p-type pillar layers and n-type pillar layers buried in a drift layer laterally and alternately (for example, JP-A 2003-273355). In the superjunction structure, the quantities of charges (the quantities of impurities) contained in the p-type pillar layers and the n-type pillar layers are equalized to artificially create a non-doped layer. This is effective to retain a high breakdown voltage and cause a current flowing through highly doped, n-type pillar layers, thereby realizing a low on-resistance that exceeds the material limit.
The breakdown voltage of the MOSFET having such the superjunction structure increases in proportion to the thickness of the superjunction structure. Therefore, realization of a high-breakdown voltage device requires a thick superjunction structure. In general, the superjunction structure is formed by a method of repeating ion implantation and epitaxial growth or a method of forming trenches in a semiconductor layer, followed by crystal growth to bury a semiconductor layer in the trenches. Accordingly, formation of a thick superjunction structure requires an increase in the number of repetitions of ion implantation and epitaxial growth, and an increase in the depth of the trench.